A method for measurement of the absolute two-photon absorption cross-section, (, of a molecule in solution has been developed. The ratio of fluorescence induced by pulsed infrared excitation to fluorescence induced by that same excitation doubled in frequency by a non-linear crystal is used to obtain an absolute value for (, eliminating the need for knowledge of the quantum yield, collection efficiency, or concentration. We have used this method in conjunction with our measurements by relative methods of molecules designed by the Marder and Perry groups at Caltech (see Measurement of Molecules with Large Two-Photon Cross-Sections) to have large ( values (5000 GM). Our measurements of Rhodamine B in MeOH agree well with Xu and Webb (1996). Large Two-Photon Cross-Sections) to have large ( values (5000 GM). Our measurements of Rhodamine B in MeOH agree well with Xu and Webb (1996). Chris Xu and Watt W. Webb, "Measurement of two-photon exitation cross-sections of molecular fluorophores with data from 690 to 1050 nm" in J. Opt. Soc. Am. B 13 (3), 481-491 (1996).